Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins. Conventional steam cracking utilizes a pyrolysis furnace wherein the feedstock, typically comprising crude or a fraction thereof optionally desalted, is heated sufficiently to cause thermal decomposition of the larger molecules. Steam is typically added to the pyrolysis furnace inter alia to reduce hydrocarbon partial pressure, to control residence time, and to minimize coke formation. Among the valuable and desirable products obtained from the furnace include light olefins such as ethylene, propylene, and butylenes. The pyrolysis process, however, also produces molecules that tend to combine to form high molecular weight materials known as steam cracked tar or steam cracker tar (“SCT”), sometimes referred to as pyrolysis fuel oil. Typically tar, as well as steam cracked gas oil (“SCGO”) is recovered as bottoms product in the first fractionator after the steam cracker. These are among the least valuable products obtained from the effluent of a pyrolysis furnace. In general, feedstocks containing higher aromatic boiling materials (“heavy feeds”) tend to produce greater quantities of SCT.
SCT is among the least desirable of the products of pyrolysis since it finds few uses. SCT tends to be incompatible with other “virgin” (meaning it has not undergone any hydrocarbon conversion process such as FCC or steam cracking) products of the refinery pipestill upstream from the steam cracker. At least one reason for such incompatibility is the presence of asphaltenes. Asphaltenes are very high in molecular weight and precipitate out when blended in even insignificant amounts into other materials, such as fuel oil streams.
The increasing use of lower quality crude feeds to the refinery, i.e., heavier, and more aromatic and/or higher sulfur feeds, has increased the amount of tar produced and, in the case of higher sulfur feeds, increased the difficulty of disposing of it. While tar has always been difficult to dispose of, the tar obtained from these heavy and/or high sulfur feeds is less compatible with refinery fuel oil pools and the typically higher sulfur levels render it unacceptable for burning.
One way to avoid production of SCT is to limit conversion of the pyrolysis feed, but this also reduces the amount of valuable products such as light olefins. Another solution is to “flux” or dilute SCT with stocks that do not contain asphaltenes, but this also requires the use of products that find higher economic value in other uses.
Methods of upgrading tar have been proposed in the prior art, but these methods are inefficient and/or do not provide sufficient volume of disposal of low value tar. For instance, U.S. Pat. No. 4,207,168 teaches making needle coke from pyrolysis fuel oil by separating quinoline insolubles and asphaltenes from the fuel oil and subjecting the remaining portion to coking.
GB 2 014 605 discloses a method of treating the pyrolysis fuel oil hydrocarbon fraction boiling at above 200° C. to solvent extraction to remove insoluble polymeric compounds. The polymer-free portion is said to be useful as fuel oil.
In the disclosure of U.S. Pat. No. 4,309,271, hydrocarbons are subjected to hydrogenation and, after separation of the product into liquid and gaseous fractions, the liquid fraction is cracked and fractionated. A polymer free fraction of the residue is returned to the feedstock and to the hydrogenation stage, and a heavy residue component of the initial liquid fraction partially oxidized with the residue.
GB 2 104 544 discloses treating pyrolysis tar obtained from the production of ethylene from naphtha feeds via steam cracking by first heating the feedstock with hydrogen to saturate polynuclear aromatic compounds, then hydrocracking the hydrogenated compounds in a cracking zone to obtain an effluent from the cracking zone which may be separated into a gaseous and liquid product.
U.S. Pat. No. 4,548,704 relates to making pitch suitable for spinning into carbon fibers, the pitch being derived from a deasphaltenated middle fraction of a feedstock.
Despite these advances, there remains a problem that SCT continues to be generated in amounts beyond the capacity of current technology to be efficiently utilized. Thus, significant amounts of SCT must be disposed of by adding to fuel oil pools or simply local combustion to generate, for example, steam. However, steam cracker tar, even relatively low asphaltene steam cracker tar, is generally incompatible with fuel oil pools such as Bunker C fuel oil. Onsite tar burning in site boilers is then preferred to avoid tar separation investment, but tighter emission regulations increasingly limit the amount that can be burned for this purpose.
Since at least the early 1980s, the bottoms of the primary fractionator downstream of a pyrolysis furnace has been fed to a vacuum tower or vacuum pipestill (“VPS”), resulting in the production of a heavy tar asphaltenic product as bottoms of the VPS. However, the quantity of this heavy tar asphaltenic product was very small and could be disposed of by blending, optionally with a fluxant, into various fuel oil pools such as Bunker fuels, or by local combustion to generate steam. However, SCT is now being generated in amounts beyond the capacity of current technology to be efficiently utilized, because of the general incompatibility of steam cracker tar, even relatively low asphaltene steam cracker tar, with fuel oil pools. Onsite tar burning in site boilers, an alternative to blending used to avoid tar separation investment, is generally precluded by tighter emission regulations increasingly limit the amount that can be burned for this purpose.
A gas/liquid “cyclone” separator was described by Van Dongen and Ter Linden for oil refining in Transactions of the ASME, January 1958, pp. 245-251.
In U.S. Pat. No. 4,140,212, a distillation tower is described including a tangential inlet and cooperating internal baffles for creating a whirling flow pattern, with a means for recovery of hydrocarbons from waste oil introduced to the tower through the tangential inlet.
The present inventors have discovered that feeding tar from a pyrolysis furnace to a vacuum pipestill equipped with an annular ring device such as the entrainment devices taught in the aforementioned ASME article and/or U.S. Pat. No. 4,140,212, to minimize entrainment of liquid in the vapor phase going overhead provides an efficient method of reducing or eliminating the problem of disposal of steam cracker tar.